The opt_footnote_marks() function

Use sza to create a gt table, adding three footnotes. Call opt_footnote_marks() to specify which footnote marks to use.

sza |>
  dplyr::filter(latitude == 30) |>
  dplyr::group_by(tst) |>
  dplyr::summarize(
    SZA.Max = if (
      all(is.na(sza))) {
      NA
    } else {
      max(sza, na.rm = TRUE)
    },
    SZA.Min = if (
      all(is.na(sza))) {
      NA
    } else {
      min(sza, na.rm = TRUE)
    },
    .groups = "drop"
  ) |>
  gt(rowname_col = "tst") |>
  tab_spanner_delim(delim = ".") |>
  sub_missing(
    columns = everything(),
    missing_text = "90+"
  ) |>
  tab_stubhead(label = "TST") |>
  tab_footnote(
    footnote = "True solar time.",
    locations = cells_stubhead()
  ) |>
  tab_footnote(
    footnote = "Solar zenith angle.",
    locations = cells_column_spanners(
      spanners = "spanner-SZA.Max"
    )
  ) |>
  tab_footnote(
    footnote = "The Lowest SZA.",
    locations = cells_stub(rows = "1200")
  ) |>
  opt_footnote_marks(marks = "standard")

TST*	SZA†
	Max	Min
0400	90+	90+
0430	90+	90+
0500	90+	90+
0530	88.9	84.7
0600	87.2	78.7
0630	87.5	72.5
0700	89.4	66.3
0730	83.7	60.0
0800	78.3	53.5
0830	73.2	47.1
0900	68.4	40.6
0930	64.1	34.1
1000	60.4	27.7
1030	57.3	21.2
1100	55.0	15.1
1130	53.5	9.6
‡ 1200	53.0	6.9
* True solar time.
† Solar zenith angle.
‡ The Lowest SZA.